This proposal seeks to develop a cancer diagnostic based on microRNA (miRNA) expression patterns found in human serum. Mounting evidence is showing that patterns of miRNA expression and the appearance of these signals in circulating extra-cellular compartments are hallmarks of proliferating tumor cells. Our preliminary results have shown a clear ability to distinguish sera derived from cancer patients for 5 different types of cancer. In a data set involving 246 patient-derived serum samples, we were able to distinguish cancer derived sera from normal controls with a sensitivity of 76% and a specificity of 84%. These data, though promising, also indicated to us that a more sensitive assay is necessary in order to resolve these distinct patterns of miRNA expression. To this end, we propose the development of a multiplexed padlock probe assay, integrated with microarray detection that will simultaneously detect most of the known human miRNAs. Our unique customizable DNA microarray synthesis platform allows us to both produce the padlock probes and to detect signal from the amplified mixture. Finally, using this assay, we will investigate the extracellular compartment or structure that protects the miRNA molecules that are secreted by proliferating tumor cells. In phase II, we will apply this assay to look for patterns of miRNA and other non-coding RNA (ncRNA) expression from normal and cancer patient serum samples, repeating our previous work with this novel assay.